It is frequently necessary in industrial operations to continuously and accurately measure the moisture concentration of process streams, e.g., hydrocarbon streams, particularly natural gas streams where the moisture is present as a vapor. In The Permutit Company's U.S. Pat. No. 4,589,971 by Donald P. Mayeaux which was issued on May 20, 1986 there is disclosed an instrument, or analyzer for the continuous and accurate measurement, in natural gas streams, of moisture concentrations measured in parts per million (ppm), based on the total volume (or weight) of the gas. The analysis for water vapor is made in the presence of contaminants, e.g., glycols, methanolamine, iron oxide particulates and methanol antifreeze which is often injected into natural gas lines.
Permutit's moisture analyzer, and some earlier moisture analyzers such as that described in U.S. Pat. No. 2,830,945 to F. E. Keidel (over which Permutit's moisture analyzer is an improvement), utilizes a cell which contains a pair of electrodes covered by a hydroscopic electrolyte, which when contacted by a moisture-containing sample will absorb moisture and produce electrolysis, and an electrolytic sensor for detecting as output from the cell an electric current as a function of the amount of moisture contained in the gas passed through the cell. The Permutit cell is characterized as a compartmented structure, a first compartment into which a moisture-containing gas specimen can be admitted, via an inlet, and expelled via an outlet, and a second compartment separated from the first compartment by a semi-permeable membrane through which moisture can be passed. The second compartment contains an electrical circuit which includes at least one pair of electrically isolated electrodes located in close proximity one to another on which can be disposed a hydroscopic substance, or electrolyte, e.g., P.sub.2 O.sub.5, which is electrically conductive when wet and in contact with both electrodes. A direct current power source of voltage is provided which is sufficient to electrolyze the electrolyte when it is wetted by moisture passed from the specimen or sample through the semi-permeable membrane from the first compartment to said second compartment, as well as current measuring means for measuring the water content of the electrolyte as a function of the current delivered by the power source in electrolyzing the electrolyte. Water molecules dependent upon their concentration within the sampled fluid passing through the first compartment, are transported through the membrane to the electrolyte. The water is disassociated, or electrolytically decomposed to its elemental components, hydrogen and oxygen, by action of the electrical current upon the electrolyte. An electrical signal is produced which is directly proportional to the number of water molecules which are transported through the membrane, and this signal in turn is directly related to the moisture, or water vapor concentration within the sampled gas. Hence, this signal is readily measured and converted electrically to a readout representative of the moisture concentration within the sampled gas. Unlike previous instruments of this type, the dependency upon sample flow rate is eliminated, and replaced by a far more accurate type of physical measurement. Moreover, the separation of the electrolyte from direct contact with the sampled fluid due to the presence of the semi-permeable membrane protects the electrolyte from solids and liquids contamination. It also prevents the electrolyte from being leached, or washed-out by slugs of liquids. The detector response for these reasons remains stable over longer periods of time.
Despite these improvements, the sensitivity of the cell gradually fades with age; albeit at a slower rate than previous cells. Contaminants, particularly those of small molecular size or low molecular weight, e.g., volatile amines, alcohols and the like, pass through the membrane and contact the electrolyte. Moreover, the electrolyte is very gradually dehydrated over a long period of time. For example, the P.sub.2 O.sub.5 electrolyte is applied as a wet orthophosphoric acid, and water is gradually removed from the electrolyte on application of a sufficiently high voltage as normally applied across the cell. Thus, the orthophosphoric electrolyte in presently used cells over a period of time is gradually reduced to the anhydride P.sub.2 O.sub.5 form which is less than one-half as active as dry orthophosphoric acid in the electrolytic decomposition of water molecules.